In recent years, many studies have been conducted on a technique for enhancing a signal coming from a specific direction using a plurality of microphones but suppressing the other sound signals, or a technique for detecting the direction of a sound source. There is a delay-and-sum array as a representative microphone array method (J. L. Flanagan, J. D. Johnston, R. Zahn and G. W. Elko, “Computer-steered microphone arrays for sound transduction in large rooms,” J. Acoust. Soc. Am., vol. 78, No. 5, pp. 1508-1518, 1985). This method is based on a principle in which, when a predetermined delay is inserted into the signal of each microphone and an adding process is performed, only the signals coming from a predetermined direction are composed in the same phase and then enhanced, but the signals coming from the other directions have different phases and are composed to have a low level. In the delay-and-sum array, the adding process is performed on the basis of this principle to enhance the signal in a specific direction. That is, directivity is formed in the specific direction. An output signal Y(t) obtained by the delay-and-sum array is represented by the following Expression (1):
                              Y          ⁡                      (            t            )                          =                              ∑                          n              =              1                        N                    ⁢                                    X              n                        ⁡                          (                              t                +                                  n                  ⁢                                                                          ⁢                  τ                                            )                                                          (        1        )            
In Expression (1), N is the number of microphones and Xn(t) is a pickup signal obtained by each microphone (n=1 to N). It is assumed that the microphones are arranged at regular intervals in the order of suffix n. In addition, τ is a delay time for making the phases of the pickup signals equal to each other in the arrival direction of a target sound.
As another example of the microphone array method, there is a Griffith-Jim type array (L. J. Griffiths and C. W. Jim, “An Alternative Approach to Linearly Constrained Adaptive Beamforming,” IEEE Trans. Antennas&Propagation, Vol. AP-30, No. 1, January, 1982). The Griffith-Jim type array is a method of removing an interference sound using an adaptive filter. For example, in the Griffith-Jim type array using two microphones, it is assumed that a target sound comes from the front of the array and an interference sound comes from the side of the array. In this case, the target sound coming from the front of the array is picked up in the same phase by the left and right microphones. As a result, the target sound is enhanced by the adding unit on the same principle as that in the delay-and-sum array. The target sound is subtracted in the same phase by a subtracting and is removed. Since the phase of the interference sound is not aligned between the microphones, the interference sound is output without being enhanced by the adding unit and being removed by the subtracting unit. It is a key point that the output signal of the subtracting unit is composed of only a so-called noise component except for the target sound. In the Griffith-Jim type array, the adaptive filter is driven using the output signal as a reference signal to remove the noise component remaining in the output of the adding unit, thereby enhancing the target sound.
In the above-mentioned array processing, it is premised that a plurality of microphones has the same sensitivity. However, in practice, the sensitivities of the microphones are different from each other and a variation in the sensitivity over time is not negligible. Therefore, it is difficult to constantly maintain the same sensitivity. When the microphones having different sensitivities are used to form an array, it is difficult to form designed directivity. For example, in the Griffith-Jim type array, the subtracting unit is used to remove the target sound. However, when two microphones have different sensitivities, a difference in amplitude remains even when the target sounds are subtracted in the same phase. The remaining difference is supplied to the adaptive filter. When the adaptive filter is used, some of the target sound components are removed from the output of the adding unit and a significant problem which causes distortion in the final output signal occurs.